The present invention relates to voltage regulating circuits and is directed more particularly to voltage regulating circuits wherein a controllable shunt reactance varies the voltage induced across an inductor, as required, to maintain that induced voltage equal to the difference in voltage between an unregulated input voltage and a regulated output voltage.
Voltage regulators utilizing series-connected inductors in association with shunt-connected capacitors have long been known and used for providing a substantially constant output voltage from an unregulated a-c input voltage. Early forms of such regulators were known as ferroresonant regulators and utilized series-connected inductors together with shunt-connected networks which included capacitors and saturable core magnetic units. Such concepts are shown, for example, in U.S. Pat. No. 2,143,745 granted to J. G. Sola on Jan. 10, 1939, and U.S. Pat. No. 2,377,152 granted in the name of H. M. Huge on May 29, 1945. Because of the excessive weight and audible noise associated with these saturable core magnetic units and because of the difficulty of obtaining an output voltage waveform of satisfactory harmonic content, many attempts have been made to improve upon ferroresonant type voltage regulators.
One attempt to improve upon ferroresonant voltage regulators has involved the substitution of a plurality of series-connected, saturable core magnetic units for each single saturable core magnetic unit thereof. such substitutions are shown, for example, in U.S. Pat. No. 3,092,768 granted in the name of A. Kusko on June 4, 1963 and U.S. Pat. No. 3,139,577 granted in the name of D. Krezek on June 30, 1964. In circuits of the latter type, the seies-connected magnetic units saturate in a predetermined sequence to generate a waveform which is approximately sinusoidal. While circuitry of this type can provide a waveform of satisfactory harmonic content, it increases the complexity and cost of the regulator without eliminating the problems of excessive weight and audible noise.
Another attempt to improve ferroresonant voltage regulators has involved the substitution of gate controlled switching devices and linear inductances for each saturable core magnetic unit. Such substitutions are shown, for example, in U.S. Pat. No. 3,076,924 granted in the name of E. W. Manteuffel on Feb. 5, 1963. In such circuits, the linear inductance simulated the saturated impedance of a saturable core magnetic unit and the gate controlled switching devices simulated off-on conducting characteristic thereof. While the utilization of such solid-state circuitry did reduce the weight and noise associated with ferroresonant voltage regulators, it did not solve the problem of reducing the high harmonic content of its output voltage. In addition, such solid-state circuitry did not lend itself to use in true three phase voltage regulators. As a result, polyphase forms of such solid-state regulator circuits were produced by the uneconomical expedient of coupling together a plurality of single-phase solid-state regulator circuits.
A still more recent attempt to solve the above-described problem is described in U.S. Pat. No. 3,745,437 granted in my name on July 10, 1973. The circuitry shown and described in that patent will hereinafter be referred to as the circuitry of my earlier patent. While the circuitry of my earlier patent solves the problem of providing a true polyphase regulator circuit having an output voltage of acceptable harmonic content, it does not satisfactorily deal with certain practical problems which prevent that circuitry from being economical. In the circuitry of my earlier patent, it is, for example, desirable to utilize relatively expensive circuit components having special operating characteristics. Thyristors which can turn on and off in very short times are typically necessary. High performance core materials and low inductance capacitors are also typically necessary. Thus, while the circuitry of my earlier patent is a useful and operative structure, it has certain practical deficiencies. An additional disadvantage of the circuitry described in my earlier patent is that it produces substantial amounts of audible noise. This audible noise, in turn, imposes further practical limitations on the usefulness of the circuitry since the magnetic units must ordinarily be potted and the circuit as a whole surrounded by sound absorbing materials.
In accordance with the present invention, there is provided polyphase regulating circuitry which exhibits all of the numerous advantages of the regulating scheme described in my earlier patent, which is realizable with magnetic units utilizing ordinary core materials and ordinary winding configurations, and which utilizes ordinary capacitors and ordinary semiconductor devices. In addition, the regulating circuit of the present invention eliminates the problem of excessive audible noise and produces an output waveform of harmonic content which is even lower than that produced by the circuit of my earlier patent. Thus, the circuit of the present invention is a significant improvement over the voltage regulator circuit of my earlier patent.